This invention relates to lasers and ways of tuning them.
A typical laser arrangement has an active laser medium or laser gain element in an optical cavity bounded by a 100% reflector on one end and a partial reflector on the output end. The gain element contains for example, an organic "dye" which fluoresces or scintillates when stimulated or "pumped" by an outside light source which flashes briefly. The scintillation then reflects briefly between the end reflectors and stimulates the dye each time it passes through until a sudden burst of laser energy is emitted which passes through the output mirror at the output end. This type of laser is referred to as a pulsed laser, rather than a continuous wave type of laser.
A variety of lasers, such as dye lasers, contain laser gain elements which emit laser light over a range of wavelengths. In many applications it is desirable to limit the laser light output from the device to one wavelength, and further, to rapidly select that wavelength, in other words, to rapidly tune the laser.
Early tuning devices were mechanical in that physical movement was required to change output wavelength. For example, the back reflector in the optical resonant cavity of a laser can be replaced with a rotatable diffraction grating. The grating reflects only one wavelength of light back through the cavity and out the output mirror. By rotating the grating to a different angle, a different wavelength of light is reflected to the output. To achieve more rapid tuning, devices were developed in which rather than rotating the diffraction grating, the angle of light incident on the grating was changed, thereby eliminating the mechanical step.
Acousto-optic (A-O) cells are among the materials which have been used to change this angle of incident light. An A-O cell is one which is transparent to light and will also propagate an acoustic, or compression, wave which will interact with the light. In one technique, light passes through the cell at an angle nearly perpendicular to its length. A flat piezoelectric crystal is attached to one end of the cell. This crystal will vibrate the cell and send acoustic waves through the cell in response to an electrical signal applied to the crystal. When light passing through the cell in one direction meets acoustic waves traveling crosswise, the light appears to be reflected from the acoustic waves, a phenomenon known as Bragg reflection. The light leaving the cell is thus diffracted-it is deflected at an angle (a "diffraction angle") and this angle is proportional to the wavelength of the light, thereby tending to fan out multichromatic light. Attention is directed to E. I. Gordon, Applied Optics Vol. 5, p. 1629 for a more complete discussion of acousto-optic cells.
Prior art devices use the change in light direction to change the angle of incidence of the light on a diffraction grating. The diffraction grating acts as a dispersive element in that it disperses the light into its frequency components. A tuning device is created by arranging the grating so that only one wavelength, determined by the angle of incidence from the A-O cell, is reflected back through the cavity to the output.
The angle of diffraction of the A-O cell is changed by changing the frequency of the acoustic wave in the cell. This in turn tunes the laser to different wavelengths.
A disadvantage of using a diffraction grating is that some light is scattered and lost from the system.